United States railroads, especially the Class 1 railroads (BN, UP, CSX, NS, CP and CN) are demanding higher hardness levels and deeper hardness in the head of railroad rail for improved in-track life (higher hardness gives better wear resistance). The American Railway Engineering and Maintenance-of-Way Association (AREMA) is one of the recognized organizations for promulgating rail specifications in North America. There are three types of AREMA rail steel based on minimum properties: standard strength, intermediate strength, and high strength. The minimum properties for each steel type are set forth in the table below:
PropertyStandardIntermediateHighSpecifiedStrengthStrengthStrengthHardness, Brinell HB (HRC)310 (30.5)325 (32.5)370 (38.3)Yield strength, ksi7480120Tensile strength, ksi142.5147171Elongation (in 2″), %10810
The hardness is specified in the rail head only. The above properties as reported and measured herein are tested according to AREMA standards set forth in AREMA Part 2, Manufacture of Rail (2007). To meet the AREMA standards of high strength, the rail must have a fully pearlitic microstructure with substantially no untempered martensite allowed. Generally, the elongation should be 10% or higher for high strength rail steel, although a relatively small number (e.g., about 5 percent) of rails may have an elongation less than 10% but no lower than 9%.
The most difficult grade to produce is the high strength grade. Some rail producers strive to achieve the required properties of high strength steel through accelerated cooling of the rail directly in-line after the rolling mill. Other producers reheat the rail from ambient temperature and then apply accelerated cooling (an off-line process). The process of cooling the rail is called head hardening. In the United States, the currently practiced cooling processes use either water sprays to cool the rail or high volume air manifolds. In all the head hardening processes the rail is cooled at a moderate cooling rate to form a fine pearlitic microstructure and to avoid the formation of untempered martensite which is not allowed by AREMA.
In addition to accelerated cooling to develop a fine pearlite interlamellar spacing, it is known to add alloying elements to the rail steel to increase hardness. Traditionally for the past decade, it has been known in the United States to use high strength head-hardened steel containing 0.80-0.84 wt % C, 0.80-1.1 wt % Mn, 0.20-0.40 wt % Si and 0.20-0.25 wt % Cr. The high carbon level of 0.80-0.84 wt % provides the pearlitic microstructure and at this carbon level the steel is at or slightly above the eutectoid point of the iron-carbon binary phase diagram. Carbon is essential because the pearlitic microstructure that develops contains about 12 wt % iron carbide (cementite) in the form of platelets imbedded alongside platelets of ferrite (forming a lamellar morphology). The cementite platelets provide hardness and wear resistance.
It has long been known that further increases in carbon can provide increased hardness of pearlite as the volume fraction of the hard cementite phase increases. When steel has a carbon level that is above the eutectoid point, however, cementite may form on the prior austenitic grain boundaries. This form of cementite is called proeutectoid cementite and the steel is referred to as hypereutectoid steel. Reduced ductility may occur in hypereutectoid steels if a continuous proeutectoid cementite network develops on the prior austenitic grain boundaries, rendering the steel brittle and unacceptable as a railroad rail.